Research On Fungi Assisted Preparation Of Cottonseed Shell Biochar And Its Performance In Supercapacitors

Supercapacitors have received much concern for their rapid charge-discharge rate,long cycle life and high power density.As an integral part of supercapacitor,electrode is a significant factor for improving the performance of supercapacitor.Due to the stable structure and good conductivity,carbon materials occupy a dominant position in the electrode materials of supercapacitors.Therein,biochar is an ideal choice because of its wide range of precursors,rich pores and surface functional groups.Biochar is usually obtained from biomass through pretreatment,carbonization and activation.The pretreatment of biomass with hyphae that is produced in the process of fungi growth can improve the pore structure of biomass and promote the effect of activation,which is conducive to improving the specific capacitance of biochar.However,the conventional way of fungi pretreatment takes a long time,which not only causes excessive degradation of biomass,but also leads to a long material preparation cycle.As a result,the ion transport capacity and graphitization degree of biochar decreases,which has an adverse impact on rate performance of biochar.In view of the above problems,this paper has carried out two parts of research work:(1)In this work,cottonseed shell biochar is prepared by fungi degradation of lignin and KOH activation,and used for supercapacitor electrode.By adjusting the pretreatment time and activation conditions of fungi,the rate performance of supercapacitor was effectively improved.A precusor of agricultural waste cottonseed shell is pretreated by Pleurotus ostreatus hyphae first,and then is carbonized and activated with KOH to prepare biochar.It is found that the cottonseed shell biochar obtained by pretreatment of Pleurotus ostreatus hyphae for 28 days and activation at carbon/KOH mass ratio of 1:4and 800°C has good electrochemical properties.In 6 mol/L KOH,the specific capacitance of cottonseed shell biochar can reach 298 F/g at 0.5 A/g,which is benefited from the specific surface area of 3072 m~2/g.Meanwhile,due to the short fungi pretreatment time,the cottonseed shell biochar has more mesopores,which is conducive to ion transport.Therefore,it still keep a capacitance retention of 79.83%at 20 A/g.The results show that although the fungi pretreatment time is only 28 days,it can still effectively promote the KOH activation process,leading to a high specific surface area in cottonseed shell biochar,and effectively improve the pore size distribution of biochar,so as to improve the rate performance.(2)In this work,the degradation of lignin and the adsorption of iron salt by fungi,together with the catalytic graphitization of iron salt and KOH activation,are used to prepare cottonseed shell biochar for supercapacitor e lectrode.The rate performance of supercapacitor is effectively improved by regulating the concentration of iron salt.Firstly,the iron salt is introduced into cottonseed shell,and then the iron containing cottonseed shell is pretreated by hyphae of Pleurotus ostreatus,and then is carbonized and activated with KOH to prepare biochar.Iron can promote the formation of graphite crystal structure at low heating temperature.Increasing the graphitization degree of carbon material is an effective way to improve its rate performance.The growth of Pleurotus ostreatus hyphae in cottonseed shell and the absorption of iron can promote the uniform distribution of iron salt in cottonseed shell,which is beneficial to improving the graphitization degree and uniformity of cottonseed shell biochar.It is found that although the existence of iron will inhibit the growth of hyphae,the final cottonseed shell biochar still has a high surface area(2854 m~2/g)as well as reasonable microporous and mesoporous structure,and its graphitization degree has been significantly improved.In 6 mol/L KOH,the capacitance retention rate of cottonseed shell biochar assisted by iron salt and fungi at 20 A/g increases to 87.09%.In the symmetrical capacitor assembled by cottonseed shell biochar,the capacitance retention rate reaches 86.78%at 2 A/g,and the energy density reaches34.1 Wh/kg and 27.0 Wh/kg when the power density is 1.2 k W/kg and 4.5 k W/kg,respectively.In addition,the specific capacitance does not decrease during 3600 cycles of charge and discharge.The results show that the rate performance of cottonseed shell biochar can be significantly improved by co-treatment of fungi and iron salt.